Lisa Oliver

The N-terminal end of bax contains a mitochondrial-targeting signal

The translocation of Bax α, a pro-apoptotic member of the BCL-2 family from the cytosol to mitochondria, is a central event of the apoptotic program. We report here that the N-terminal (NT) end of Bax α, which contains its first α helix (Ηα1), is a functional mitochondrial-addressing signal both in mammals and in yeast. Similar results were obtained with a newly described variant of Bax called Bax ψ, which lacks the first 20 amino acids of Bax α and is constitutively associated with mitochondria. Deletion of Ηα1 impairs the binding of Bax ψ to mitochondria, whereas a fusion of the N terminus of Bax α, which contains Ηα1 with a cytosolic protein, results in the binding of the chimeric proteins to mitochondria both in a cell-free assay and in vitro. More importantly, the mitochondria-bound chimeric proteins inhibit the interaction of Bax ψ with mitochondria as well as Bax-apoptogenic properties. The mutations of the Ηα1, which inhibit Bax α and Bax ψ translocation to mitochondria, also block the subsequent activation of the execution phase of apoptosis. Conversely, a deletion of the C terminus does not appear to influence Bax α and Bax ψ mitochondrial addressing. Taken together, our results suggest that Bax is targeted to mitochondria by its NT and thus through a pathway that is unique for a member of the BCL-2 family.

Bax activation by the BH3-only protein Puma promotes cell dependence on antiapoptotic Bcl-2 family members

It is still unclear whether the BH3-only protein Puma (p53 up-regulated modulator of apoptosis) can prime cells to death and render antiapoptotic BH3-binding Bcl-2 homologues necessary for survival through its ability to directly interact with proapoptotic Bax and activate it. In this study, we provide further evidence, using cell-free assays, that the BH3 domain of Puma binds Bax at an activation site that comprises the first helix of Bax. We also show that, in yeast, Puma interacts with Bax and triggers its killing activity when Bcl-2 homologues are absent but not when Bcl-xL is expressed. Finally, endogenous Puma is involved in the apoptotic response of human colorectal cancer cells to the Bcl-2/Bcl-xL inhibitor ABT-737, even in conditions where the expression of Mcl-1 is down-regulated. Thus, Puma is competent to trigger Bax activity by itself, thereby promoting cellular dependence on prosurvival Bcl-2 family members.

Induction of a Caspase-3-like Activity by Calcium in Normal Cytosolic Extracts Triggers Nuclear Apoptosis in a Cell-free System

Calcium is involved in several steps of the apoptotic process. In nuclei, endonucleases are presumed to be the main targets of calcium; however, little is known about its role during the cytosolic phase of apoptosis. We used a cell-free system to address this question. Our results show that CaCl2 triggered nuclear apoptosis (i.e. typical morphological change and DNA fragmentation) at concentrations of 5 mm. This concentration was lowered 10-fold by the co-incubation with cytosolic extracts from nonapoptotic cells. Apoptotic changes induced by the incubation of nuclei with CaCl2 in the presence of these cytosols were strongly reduced in the presence of an inhibitor of caspase-3 and to a lesser extent by an inhibitor of caspase-1. We also show that calcium-induced apoptosis is affected by protease inhibitors such as N-tosyl-l-phenylalanine chloromethyl ketone, but not by calpain or several lysosomal protease inhibitors. The addition of CaCl2 to the cell-free system increased a caspase-3 activity in nonapoptotic cytosols as shown by specific antibodies and an enzymatic assay. No activation of a caspase-3-like activity by the addition of cytochrome c was observed in these extracts under similar conditions. The enhanced caspase-3 activity induced by calcium was inhibited by protease inhibitors affecting morphological nuclear apoptosis except for those responsible for the degradation of lamin A. These results suggest that CaCl2 could trigger, in normal cells, an apoptotic cascade through the activation of cytosolic caspase-3 activity.

Nonredundant Role of Bax and Bak in Bid-Mediated Apoptosis

ABSTRACT Animal models suggest that Bax and Bak play an essential role in the implementation of apoptosis and as a result can hinder tumorigenesis. We analyzed the expression of these proteins in 50 human glioblastoma multiforme (GBM) tumors. We found that all the tumors expressed Bak, while three did not express Bax. In vitro, Bax-deficient GBM (BdGBM) exhibited an important resistance to various apoptogenic stimuli (e.g., UV, staurosporine, and doxorubicin) compared to the Bax-expressing GBM (BeGBM). Using an antisense strategy, we generated Bak− BeGBM and Bak− BdGBM, which enabled us to show that the remaining sensitivity of the BdGBM to apoptosis was due to the overexpression of Bak. Bax/Bak single or double deficiency had no influence on either the clonogenicity or the growth of tumors in Swiss nude mice. Of note, Bak− BeGBM cells were resistant to apoptosis induced by caspase 8 (C8) but not to that induced by granzyme B (GrB). Cells lacking both Bax and Bak (i.e., Bak− BdGBM) were completely resistant to all stimuli including the microinjection of C8 and GrB. We show that GrB-cleaved Bid and C8-cleaved Bid differ in size and utilize preferentially Bax and Bak, respectively, to promote cytochrome c release from mitochondria. Our results suggest that Bax deficiency is compensated by an increase of the expression of Bak in GBM and show, for the first time in human cancer, that the double Bax and Bak deficiency severely impairs the apoptotic program.

Distinct Domains Control the Addressing and the Insertion of Bax into Mitochondria

The translocation of Bax from the cytosol into the mitochondrial outer membrane is a central event during apoptosis. We report that beyond the addressing step, which involves its first α-helix (hα1), the helices α5 and α6 (hα5α6) are responsible for the insertion of Bax into mitochondrial outer membrane bilayer. The translocation of Bax to mitochondria is associated with specific changes in the conformation of the protein that are under the control of two prolines: Pro-13, which controls the unfolding of hα1, and Pro-168, a proline located immediately before the hydrophobic carboxyl-terminal end (i.e. helix α9, hα9), which controls the disclosure of hα5α6. An additional step, the disruption of an electrostatic bond formed between Asp-33 (hα1) and Lys-64 (BH3), allows the mitochondria addressing of Bax. We conclude that, although the intramolecular interactions of hα1 with the BH3 region control the addressing of Bax to mitochondria, the Pro-168 is involved in the control of its membrane insertion through hα5α6.

Melphalan-induced apoptosis in multiple myeloma cells is associated with a cleavage of Mcl-1 and Bim and a decrease in the Mcl-1/Bim complex

Multiple myeloma (MM) is a rapidly fatal plasma-cell malignancy that evolves mainly in the bone marrow. Melphalan is widely used to treat patients with MM but as yet its mechanisms of action are poorly documented. In the current study, we demonstrate that melphalan induces a drastic downregulation of Mcl-1L, Bcl-xL and BimEL in human melphalan-sensitive myeloma cells while the most potent proapoptotic isoforms, BimL and S, are affected to a lesser extent. Moreover, Mcl-1L and BimEL disappearance is associated with the generation of proapoptotic cleaved forms generated by a caspase cleavage. In myeloma cells, we have previously shown that Mcl-1 neutralizes the proapoptotic function of Bim and therefore, prevents the activation of death effectors. In this study, we demonstrate that melphalan disrupts the Mcl-1/Bim complex whereas the Bcl-2/Bim complex is not modified. The disappearance of full length Mcl-1 allows the release of Bim isoforms, particularly L and S, which can exert their proapoptotic function and leads to Bax activation and cytochrome c release. Thus, we can hypothesize that the cleaved 26 kDa proapoptotic Mcl-1 and the 19 and 12 kDa of Bim, generated during melphalan treatment could contribute to the amplification loop of apoptosis.

Involvement of the N-terminus of Bax in its intracellular localization and function

We have identified, using site‐directed mutagenesis, a proline located at position 13 of Baxα (Bax) as crucial for the maintenance of its cytosolic conformation. The substitution of this proline by a valine results in a strong binding of Bax to mitochondria and to conformational changes monitored by a decreased sensitivity of Bax to mild proteolysis and the enhancement of its oligomerization state. Deletion of the C‐terminus of Bax does not modify its intracellular localization. On the other hand, the pro‐apoptotic activity of Bax is enhanced by a deletion of the C‐terminus in the absence of the N‐terminus but is decreased in its presence. These results suggest that both extremities functionally interact to control the activity but not the subcellular localization of Bax.

Basal autophagy decreased during the differentiation of human adult mesenchymal stem cells.

Autophagy plays an important role in homeostasis, development, and disease, functioning both as a survival and cell death pathway. However, despite its importance in cell physiology, there is little information about the role of autophagy in stem cells and, in particular, on its implication in their survival and/or cell death. We describe here that in vitro, human mesenchymal stem cells (hMSCs) exhibited a high level of constitutive autophagy. Inhibitors of autophagy such as Bafilomycin A1 (Baf-A1) inhibited the proteolytic degradation associated with autophagy in these cells. In addition, we show that a knockdown in the expression of Bcl-xL is accompanied by a loss of autophagic proteolytic ability. Indeed, Bcl-xL seems to exert a tight control on autophagy regulation, since its reintroduction by a protein construct PTD-Bcl-xL resulted in the reacquisition of autophagy. We show that the suppression of autophagy through the knockdown of Bcl-xL influenced hMSC survival and differentiation. This study expands our knowledge on the control exerted by Bcl-xL on autophagy and illustrates the important role of autophagy in the maintenance and differentiation of adult hMSCs.

The Neurotrophic Activity of Fibroblast Growth Factor 1 (FGF1) Depends on Endogenous FGF1 Expression and Is Independent of the Mitogen-activated Protein Kinase Cascade Pathway

The expression of fibroblast growth factor (FGF) 1, a potent neurotrophic factor, increases during differentiation and remains high in adult neuronal tissues. To examine the importance of this expression on the neuronal phenotype, we have used PC12 cells, a model to study FGF-induced neuronal differentiation. After demonstrating that FGF1 and FGF2 are synthesized by PC12 cells, we investigated if FGF1 expression could be a key element in differentiation. Using the cell signaling pathway to determine the effects of FGF1 alone, FGF1 plus heparin, or a mutated FGF1, we showed an activation to the same extent of mitogen-activated protein (MAP) kinase kinase and MAP kinase (extracellular regulated kinase 1). However, only FGF1 plus heparin could promote PC12 cell differentiation. Thus, the MAP kinase pathway is insufficient to promote differentiation. Analysis of the PC12 cells after the addition of FGF1 plus heparin or FGF2 demonstrated a significant increase in the level of FGF1 expression with the same time course as the appearance of the neuritic extensions. Transfection experiments were performed to enhance constitutivly or after dexamethasone induction the level of FGF1 expression. The degree of differentiation achieved by the cells correlated directly with the amount of FGF1 expressed. The MAP kinase pathway did not appear to be involved. Interestingly, a 5-fold increase in FGF1 in constitutive transfected cells extended dramatically their survival in serum-free medium, suggesting that the rise of FGF1 synthesis during neuronal differentiation is probably linked to their ability to survive in the adult. All of these data demonstrate that, in contrast to the MAP kinase cascade, FGF1 expression is sufficient to induce in PC12 cells both differentiation and survival. It also shows that auto- and trans-activation of FGF1 expression is involved in the differentiation process stimulated by exogenous FGFs through a new pathway which remains to be characterized.

Comparison of spheroids formed by rat glioma stem cells and neural stem cells reveals differences in glucose metabolism and promising therapeutic applications.

Background: Cancer stem cells (CSC) are responsible for tumor resistance and relapse. Results: Spheroids formed by CSC and neural stem cells (NSC) were compared by proteomics and functional studies. DCA, a glycolytic modulator, sensitizes CSC but not NSC to apoptosis. Conclusion: DCA can eradicate glioma stem cells. Significance: DCA could be a new efficient anti-CSC treatment in glioma. Cancer stem cells (CSCs) are thought to be partially responsible for cancer resistance to current therapies and tumor recurrence. Dichloroacetate (DCA), a compound capable of shifting metabolism from glycolysis to glucose oxidation, via an inhibition of pyruvate dehydrogenase kinase was used. We show that DCA is able to shift the pyruvate metabolism in rat glioma CSCs but has no effect in rat neural stem cells. DCA forces CSCs into oxidative phosphorylation but does not trigger the production of reactive oxygen species and consecutive anti-cancer apoptosis. However, DCA, associated with etoposide or irradiation, induced a Bax-dependent apoptosis in CSCs in vitro and decreased their proliferation in vivo. The former phenomenon is related to DCA-induced Foxo3 and p53 expression, resulting in the overexpression of BH3-only proteins (Bad, Noxa, and Puma), which in turn facilitates Bax-dependent apoptosis. Our results demonstrate that a small drug available for clinical studies potentiates the induction of apoptosis in glioma CSCs.